1. Field of the Invention
The present invention relates to a solid electrolytic capacitor and a method for producing the same. More specifically, the present invention relates to a solid electrolytic capacitor obtained by stacking valve-acting metal substrates each having a dielectric film, which has remarkably reduced equivalent series resistance (ESR), and also relates to a production method therefor.
2. Description of the Related Art
With recent progress of small-size and high-frequency electronic equipment, there is a demand for a small size solid electrolytic capacitor as one of constituent electronic parts therefor. This requirement for downsizing is generally met by a chip-form stacked capacitor.
FIG. 1 is a perspective view showing a conventional chip-form solid electrolytic capacitor. A plurality of solid electrolytic capacitor elements 1 are disposed to lie in the same direction inside an outer jacket resin 6. The anode part 3 of the capacitor element 1 and the bottom surface of the cathode part 2 formed on the surface of the element are placed on an anode lead part 5 (as an anode lead pulling out part of a lead frame) and on a cathode lead part 4 (as a cathode lead pulling out part), respectively. Also, each is bonded to the part with an electrically conducting material such as an electrically conductive adhesive. The anode lead part and the cathode lead part are paired and disposed to face each other. The thus-fabricated device is molded with a separately prepared outer jacket resin 6 such as epoxy resin.
For example, in the case where the capacitor element in a solid electrolytic capacitor has a tabular shape, a dielectric film is provided on the surface of an electrode material composed of a tabular metal having valve action, a solid electrolyte layer is provided on the dielectric film, an electrically conducting cathode layer is provided on the solid electrolyte layer to form a cathode part of the capacitor element, an anode part is integrally provided on the electrode material of the capacitor element, a resist film for masking is applied to provide a portion which becomes the anode lead pulling out part, a plurality of capacitor elements are stacked one on another such that the connection part of electrically conducting cathode layers and the connection part of the anode parts come to respective corresponding positions, and the stacked body is connected to a cathode lead part and an anode lead part. By employing such a structure, the volume efficiency (capacitance value of capacitor with a fixed volume) of the capacitance of a capacitor is elevated.
The equivalent series resistance of a solid electrolytic capacitor is governed mainly by the intrinsic resistance component of the anode lead part, the contact resistance component on the joined face of the anode lead part and the anode part of the capacitor element, the intrinsic resistance component of the capacitor element, the contact resistance component on the joined face of the cathode part of the capacitor element and the cathode lead part, and the intrinsic resistance component of the cathode lead part.
In a solid electrolytic capacitor having a stacked structure, the first layer capacitor element and the cathode lead part are electrically connected with an electrically conducting material (e.g., electrically conducting adhesive). The second layer capacitor element is bonded to the first layer capacitor element using an electrically conducting material (e.g., electrically conducting adhesive) and therefore, electrically connected to the cathode lead part through the electrically conducting cathode layer formed on the surface of the first layer capacitor element. The resistance component of this electrically conducting cathode layer gives rise to a problem of increasing the equivalent series resistance of the solid electrolytic capacitor.
If the electrically conducting adhesive or the like is coated after stacking the capacitor elements to electrically connect each of stacked capacitor elements directly to the cathode lead part, the equivalent series resistance of the solid electrolytic capacitor decreases However, a step of coating the electrically conducting adhesive over respective capacitor elements is necessary and this causes a problem in productivity and profitability.
The equivalent series resistance of a solid electrolytic capacitor can also be decreased by directly joining a cathode lead part to respective capacitor elements, however, a problem is similarly present in that a complicated step of processing the cathode lead part is also necessary. For reducing the equivalent series resistance of a capacitor device, a method of boring a hole in the capacitor device has been proposed (see, JP-A-4-119624 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)), however, the equivalent series resistance between stacked capacitor elements cannot be decreased by this method.